For instance, these are the "combined" fuel economy numbers for numerous similar vehicles in Australia (all numbers converted from l/100km from GreenVehicleGuide):

Car

Engine

US MPG

L/100km

km/L

Toyota Prius C

1.5L

60

3.9

25.6

Smart ForTwo (with auto Start/Stop)

1.0L

53

4.4

22.7

Mitsubishi Mirage (no Start/Stop)

1.2L

51 - 48

4.6 - 4.9

21.7 - 20.4

Chevrolet Spark / Holden BarinaSpark

1.2L

45 - 41

5.2 - 5.8

19.2 - 17.2

Kia Rio

1.6L

42 - 39

5.6 - 6.1

17.9 - 16.4

Toyota Yaris

1.5L

41 - 35

5.7 - 6.7

17.5 - 14.9

Ford Fiesta

1.6L

39

6.1

16.4

Hyundai Accent

1.6L

39 - 37

6.0 - 6.4

16.7 - 15.6

Honda Fit

1.5L

37 - 35

6.4 - 6.7

15.6 - 14.9

According to the Australian mileage testing, only the Smart ForTwo (with start/stop) and Prius C (a hybrid) beat the Mirage out in fuel economy. That is a heck of a good start.

Sadly, the Australians don't get the Scion/Toyota iQ which currently holds the Canadian and U.S. title for having the best combined fuel economy rating for a non-hybrid (EPA: 37 mpg US combined). So we don't know yet for sure if Mirage will wear the MPG crown in the North America.

The closest comparison we really have in Australia is the Chevy (Holden Barina) Spark. To get some ballpark we can compare the ratings of the Australian Mirage and Spark to the Canadian and US Spark ratings and take a good stab at the what the Mirage's fuel economy might be.

To simplify things we'll just take the highest ratings.

Australian combined ratings:

Spark 1.2L, 4-cyl, 5-spd: 45 mpg / 5.2 L/100km / 19.2 km/L

Mirage 1.2L, 3-cyl, 5-spd: 51 mpg / 4.6 L/100km / 21.7 km/L

This shows that the Mirage is rated as having 13% better fuel economy than the Spark.

So, if this holds true (and remember, this is a best guess), the Mitsubishi Mirage could dethrone the Scion iQ for highest mileage car that is a non-hybrid! The Canadian iQ is rated at 6.5 L/100km city, 4.7 L/100km highway. In the US, the iQ is rated at 37 mpg combined, so 38 mpg would beat it.

For kicks, I looked up the Mirage's Japanese rating and it gets between 23.2 km/l / 55 mpg / 4.3 l/100km and 27.2 km/l / 64 mpg / 3.7 l/100km.

The reason I looked up the Japanese ratings is that Japan also has the Toyota/Scion iQ. It is rated at 20.8 km/l / 49 mpg / 4.8 l/100km in one trim, and 21.2 / 50 mpg / 4.7 l/100km in another. Sorry, I can't read Japanese, so I don't know the difference.

Anyway, the important thing here is that the Mirage quite easily beats the iQ in the mileage department, and in some trims by quite a bit!

Mirage beats the Scion/Toyota iQ's MPG in the UK as well

Here's another comparison for the pile that suggests the Mirage will go straight to the head of the MPG class in Canada/US (for a non-hybrid).

Again, using the Toyota iQ as the benchmark (currently the highest rated MPG for a non-hybrid in both Canada and the US):

There's a U.K. review of the Mirage that notes a 68.9 mpg (Imperial) combined rating for the 1.2L, 5-speed. The 1.3L CVT iQ (same drivetrain as in Can/US) is rated by Toyota UK at 54.32 mpg (Imperial) The Mirage apparently even beats the 1.0L manual transmission version of the iQ:

The Euro/Japan Mirage is a bit more efficient than the rest of the world (slightly higher compression, stop/start in most models, slightly better aero, slightly taller manual gearing), but the biggest difference by far is the NEDC testing method. It doesn't directly compare with the EPA/NRCAN tests -- NEDC produces unrealistically high numbers (even higher than the old NRCAN ones, but not as silly as the Japanese tests). They're currently taking a lot of flack on this topic from annoyed media & motorists.